1. The Field of the Invention
The present invention relates to hydraulic cementitious compositions, products made from such compositions, and the methods for processing such hydraulic cementitious compositions and products. More particularly, the present invention is directed to systems and processes for optimizing the performance and design properties of cementitious materials, while minimizing manufacturing costs, through a materials science approach of microstructurally engineering the materials. Further, the present invention is directed to systems and processes capable of determining the appropriate modifications to the processing parameters in a specific method of manufacture in response to variations in the feedstock materials, thereby reproducibly producing a material with consistent performance characteristics and design properties.
2. Microfiche Appendix
Submitted herewith is one (1) microfiche having a total of twenty-two (22) frames. The microfiche contains the source code for the computer-implemented design optimization process as described in the specification and drawings, and which is submitted as one example for implementing a computer program for the described process. The microfiche portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the disclosure contained herein, including any material in the disclosure that is subject to copyright protection, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
3. Technology Review
Hydraulic cementitious materials were first used about two thousand years ago by the Romans as the binding agent in mortars (i.e., now typically a combination of cement, water, and sand) and concretes (i.e., now typically a combination of cement, water, and aggregates such as sand and/or rock). This knowledge of hydraulic cementitious materials was later lost and then rediscovered in 1829 by J. Aspin in England. Since 1829, concrete has had a variety of uses because it is relatively inexpensive and can be easily worked under a wide range of conditions. Importantly, the versatility of concrete is enhanced because very little training or specialized equipment is needed to manufacture traditional concrete products. As well as having been used for more than a century as the primary material for building structures, concrete is employed in the infrastructure of every major component of modern society, e.g., pipes, sidewalks, curbs, bridges, highways, supports, foundations, and dams.
Hydraulic cement-based materials are formed by mixing cement with water to form a cement paste. Typical cement paste will have a water to cement ratio in the range from about 0.1 to about 1. As used in the specification and claims, the term "cement paste" includes a fluid mixture of cement and water. Generally, in a cement paste, the hydration reaction commences with the addition of the water, but is not completed. When water is added to the hydraulic cements, the synthesized clinker minerals in the cement reacts chemically with the water to form a new complex phase structure described as "CSH"-gel or calcium-silicate-hydrate. As a result of this reaction, the water-cement mixture sets and cures to bind the aggregates together to form mortar or concrete.
One of the most important uses of hydraulic cement compositions is in concrete. As used in the specification and claims, the term "concrete" is broadly defined as an inorganic composite material including cement paste as a primary binder that develops its properties under "near" ambient conditions. (Concrete is distinguished from inorganic ceramic materials in that it is not heated to several hundreds of degrees to develop bonding through a sintering process, rather it is a hydrated bonding material.) Concrete is a hard, strong building material made by mixing a water-cement mixture with one or more aggregates including sand, gravel, other geologic materials, metals, and/or metallic alloys.
For modern high technology concrete (for example, in the French-British tunnel or the Great Belt Link Connection in Denmark), there can be as many as 12-15 different components in the concrete mixture. Such components, as an example, may include three types of sand, three types of coarse aggregate, one specially designed cement, fly ash, silica fume, two types of plasticizing/water reducing admixtures, air entraining agents, accelerators, retarders, and water. In addition to all of these components, other variables which affect the properties of the resulting product include the processing techniques and equipment.
Typically, the two most important design criteria for cementitious materials are (a) the rheological flow properties oft he fresh concrete, and (b) the compressive strength of the concrete as measured 28 days after the beginning of the hardening process. Flow properties of concrete are typically measured by filling a 30 cm high conical cylinder with freshly mixed concrete. The conical cylinder is then removed, leaving the now conically shaped freshly mixed concrete, freestanding. The vertical distance that the concrete then drops or slumps corresponds to the flow property of the concrete. The compressive strength of concrete is typically ascertained by the load failure of concrete cylinders cured for 28 days. Strength is measured in psi (pounds/square inch) or MPa (Megapascals).
Other design criteria such as permeability, chloride diffusion (of importance to rebar corrosion and hence the durability of the whole structure), thermal cracking, drying shrinkage, plastic shrinkage, sulfate resistance, alkali silica reactions, and the number of microdefects also have a direct influence on the strength and durability performance of the concrete. Many of the above criteria are a function of the porosity of the concrete. Microdefects are normally caused by bleeding and segregation phenomena and are associated with badly designed concrete. ("Bleeding" is the phenomena where water migrates towards the surface of the concrete and collects on the surface or underneath coarse aggregates. "Segregation" is a phenomenon where the concrete has no internal cohesion, and the mortar therefore separates from the coarse aggregates.)
The same attributes that make concrete ubiquitous in application (i.e., low cost, ease of use, and wide availability of raw materials) have also kept it from being fully researched and its full potential developed and exploited. In the past, it has simply been easier to overdesign a mixture rather than try to understand and define the interrelationship and effects of the different components. As a result, the engineering properties of cement-based materials have also been limited.
After decades of experience, the craftsmanship of concrete production has developed into a system of guidelines, rules, and codes based on the empirical results of varying cement mixtures. These guidelines are an attempt to promote uniformity of concrete with desirable qualities. In the United States, the generally accepted standards for concrete design are empirically derived formulas, created by the American Concrete Institute ("ACI") Committee 211 and set forth in "Recommended Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete," ACI 211 1-81 While other countries utilize slightly different standards, the resulting formulations are substantively similar and suffer from the same deficiencies.
These empirical standards provide the concrete manufacturer at the "ready-mix plant," the "pre-mix plant," or the "construction site" with recommended amounts of cement, sand (of a particular type), coarse aggregate (of a particular type), and water to use in a concrete mixture to achieve the predetermined desired performance characteristics--mainly the flow and strength characteristics. Unfortunately, the complexity and variability of materials, environments, and applications has continued to keep cement-based materials a local trade industry based upon trial and error, rather than one based on technology and science.
The typical approach for designing a concrete mixture following ACI 211 standards is to first select a recommended mix design with a desired theoretical slump and strength. The concrete is then mixed and the actual slump determined. As a result of variables such as the size, shape, type, and range of sand, coarse aggregate, and cement, along with the mixing procedure and environment, the actual slump usually does not correspond to the theoretical slump. Accordingly, additional mixes are produced with varying amounts of water until a mix with a desired slump is obtained.
The resulting concrete is then placed in cylinders and allowed to cure for 28 days. The resulting concrete cylinders are then loaded to determine their actual compressive strength. Should the actual strength not correspond to the desired strength, the process is repeated with a new recommended mix design having a lower or higher theoretical strength, depending on the actual strength of the previous mixture. As can be readily appreciated, such a process can be very time consuming and often very costly.
This prior art process has several drawbacks. Most notably, since the process does not take into account variabilities in the components, test mixtures must be made to ensure the required slump and strength are obtained. The test mixtures cause considerable delay since at least 28 days for curing is required. Moreover, there can be a substantial expense both in terms of preparing and testing the mixture and as a result of the delay for testing. Furthermore, even when a mixture is obtained that satisfies the slump and strength requirement, there is no assurance that it is the lowest cost mixture. Additional testing may reveal that by varying the size, range, and proportions of sand, coarse aggregate, and cement, a less expensive mixture might be obtained that possesses the same or even closer desired properties of slump and strength. Finally, once an acceptable mix design for a given set of materials is obtained, it is very difficult, if not impossible, to maintain consistency of performance (i.e., the slump and strength) due to the natural variation in the raw materials.
Another design approach is to initially produce a variety of concrete mixtures by adding and varying different components including admixtures. The admixtures can include fly ash, silica fume, water reducers, pozzolans, fillers, and air entraining agents which affect the slump and strength of the concrete. The mixtures are selected from those surrounding a recommended mix design having a desired theoretical slump and strength. However, varying a concrete mixture having 13 components on 10 different experimental levels would result in 10.sup.13 total number of combinations. A laborious task!
Computer programs have been written, such as that from Shieldstone & Associate, Inc. of Dallas, Tex., which recognize these challenges and attempt to amass and sort large databases of mix designs for ascertaining an optimal mix design given a certain feedstock. In general, the Shieldstone system attempts to match particle size distributions of known concrete mixes with available feedstock so as to design mixes having similar properties. Such programs, however, have had minimal success and applicability as a result of the almost infinite types of components for a given location that can be used in a mix design.
For example, although typically not more than three types of coarse aggregate are used in a mixture, the actual size and surface texture of the types of coarse aggregate, which in turn affect the properties of the mixture, can be of almost infinite variety. Accordingly, it is extremely difficult for one having an available feedstock to match the empirical results of mixtures made from a different feedstock. The problem is compounded as the number of available components increases. Furthermore, basing a new mixture on the empirical results of a previous mixture does not improve the new mixture nor does it insure that the new mixture will be the most optimal or economical.
It is thus virtually impossible, if not utterly impractical, to use conventional testing or empirical table of past results to ascertain with confidence and accuracy what types and proportions of a multi-component mixture result in a mix design that yields a desired strength and slump and having a minimal cost for each specific batch of concrete produced. Even when a mixture is obtained that has desired strength and slump properties, there is still a question as to durability. Almost any combination of aggregate can obtain a desired strength and slump if sufficient water and cement are added. However, as the amount of water is increased, the durability of the resulting concrete structure decreases. Accordingly, use of the above processes gives no assurance that the selected mixture will be the most durable.
Attempts have also been made to model certain aspects or properties of concrete mixtures. For example, the Bolomey and Feret equations attempt to model the resulting 28-day strength of concrete and mortars, respectively. Likewise, the Larrard eguation attempts to correlate the affect of particle packing on the strength of mixtures containing cement, fly ash, and silica fume, while Popovic's formula attempts to correlate water content of a mixture to the resulting slump or workability of the mixture.
For several reasons, however, such equations have found minimal acceptance and use in the concrete industry. First, and most important, no interrelationship between the equations has been established. Thus, although the equations might be useful in estimating a specific property, independently they are of little use in designing a mixture that will accurately optimize all properties. Second, empirical studies have found that there is a deviation between the theoretical results from the above equations and the actual experimental values obtained. Finally, the equations are typically designed only for use with standard mixtures using sand, cement, and coarse aggregate and do not take into consideration the affects of air voids in a mixture or the addition of modern admixtures such as fillers, fly ash, silica fume, and other pozzolans.
The problems associated with concrete mixtures are compounded in ready mix plants where the luxury of testing a mixture before use is often not available. Due to the difficulties associated with controlling the flow behavior and loss of flow of fresh "unset" concrete over prolonged periods of time before casting (sometimes up to 10 hours after mixing), most concrete is produced in ready-mix plants located relatively close to the project site. Hence, the actual operators of the plants are trained more in evaluating the "look and feel" of the concrete material, than in designing the concrete through scientific procedures.
The prevailing practice in ready-mix plants has been to use fixed proportioning (that is, established design mixes) in a computer control system to combine these variable materials in order to obtain a cured concrete having predetermined characteristics. One of the main historical difficulties, however, in producing consistent, predictable concrete has been that the component materials used will vary from day to day and even from batch to batch or site to site. The result has been inconsistent concrete materials with high standard deviations in properties.
The concrete industry has, therefore, had to resort to a practice of overdesigning to compensate for the inability to control production consistency. A standard concrete mixture is given a theoretical design strength based on the minimum 28-day strength of test cylinders. Depending on the number of cylinders tested and the standard deviation between test results, the actual and theoretical design strength can substantially vary. In contrast, the more consistent the concrete can be produced, the less the concrete needs to be overdesigned.
The practice of overdesigning in combination with bad initial designing turns out to be more costly than might be initially apparent, both for the manufacturer and for society as a whole. When design and performance criteria are established for a specific concrete batch, the manufacturer cannot simply prepare a product that on the average meets each of those criteria. The manufacturer must design a product that will meet the minimum criteria assuming that all of the materials are simultaneously at the worst end of their range of variability.
For example, the quality and gradation of the available cement may vary in a range from A (worse) to B (best). Similarly, the quality and gradation of each type of aggregate will also vary in a range from C to D. Also, the sand quality will vary in a range from E to F. Even the quality of the water and other admixtures will vary within a given range; however, typically these will be of less importance than the cement, sand, and coarse aggregate variations.
The range for any given cement, sand, or aggregate material can be quite large since it is relatively expensive to obtain feedstock materials having a narrow range of consistent quality and size. It has been typically found to be more economical to significantly overdesign the concrete material rather than start with quality-controlled, guaranteed, consistent feedstocks.
Hence, when processing the concrete, the manufacturer must assume that at any given time, the quality of the sand is at "A" (its worst), the quality of the aggregates is at "C" (its worst), and the quality of the cement is at "E" (its worst). It becomes immediately apparent that the types and amounts of the materials actually used must be significantly different than those necessary to achieve the desired result.
The result is that a more costly product having an overdesign of upwards of several thousand psi in compressive strength may have to be produced. Nevertheless, at any given time, the product which is produced may be anywhere from just barely adequate to being overdesigned by upwards of 50%. In a highly competitive market, the results are at best a narrow, if not nonexistent, profit margin, or at worst an incentive to "cut corners" and produce an inferior product which may fail at a later date.
The process of overdesigning is also reflected in the proportions of the various components. That is, preestablished mixes typically have an excess of sand in the mixture to insure a cohesive mixture that will not bleed or segregate. The addition of excess sand, however, results in a more porous mixture that is less durable. Mixtures also typically include more cement than necessary, thereby increasing the price, in order to insure that the mixture has sufficient strength.
Furthermore, even though the ready-mix plant operator will significantly overdesign his product, the operator can never be sure that the material will meet the desired performance criteria. This is not only because of the variability in materials, but because the operators use standard mix designs (or recipes) to achieve a given set of performance parameters. These recipes are empirical and are based upon average, historical experience, but they still leave doubt as to the capabilities of any given product. The operator can never be sure whether the materials being used are precisely the same or will give the same performance as those used in developing the standard mix designs.
A further problem encountered in the day-to-day practice of the concrete industry is that, because of the above mentioned variations in the materials' properties, the truck drivers frequently take some action to symptomatically modify or "correct" the workability or flow characteristics of the concrete from those existing at the time the concrete was placed in the truck at the ready-mix plant. It has been estimated that in approximately 70% of the deliveries of concrete in the United States the truck driver will modify the concrete specification, typically by adding water to the concrete mixture to make it "pour" or "look" better. The result is that the water to cement ratio is increased and the compressive strength is decreased. In other countries, it has been determined that this practice has such serious consequences, it is not permitted. Hence, ready-mix plant production is at a further disadvantage because what happens to the concrete mix after it leaves the ready-mix plant cannot be controlled because the product is not sufficiently predictable.
From the foregoing, it will be appreciated that what is needed in the art are processes and manufacturing techniques for consistently and predictably producing uniform cementitious compositions and products which can be assured to meet predetermined quality characteristics and to meet predetermined performance criteria.
It would be another significant advancement in the art to produce consistently and predictably cementitious compositions and products which would meet the predetermined design and performance criteria, while minimizing the need to overdesign the cementitious materials and thereby minimize the cost of manufacturing.
Additionally, it would be a significant advancement in the art to produce consistently and predictably such uniform cementitious compositions and products even though feedstocks (e.g., cement, sand, gravel, aggregates, water, and admixtures) having variable qualities and attributes are utilized.
It would be yet another significant advancement in the art to provide novel compositions and processes for producing cementitious compositions and products with such predictable properties that the resultant product would not need to be modified by the truck driver or at the delivery site.
Another significant advancement in the art would be to provide novel processes for designing concrete such that the cementitious compositions predictably meet the required strength, slump, and durability characteristics.
It would be a further advancement in the art to provide novel compositions and processes for designing concrete such that trial and error approximation is eliminated.
It would be yet another advancement in the art to provide novel compositions and processes for designing concrete such that the mix design for a certain concrete having a variety of components and admixtures will be known to be optimal and at the same time be the most cost effective.
Further, it would be an advancement to provide novel processes for modifying in "real time" the manufacturing processes of cementitious compositions and products in response to changes on site of the feedstock materials.
Such cement compositions, products, and methods are disclosed and claimed herein.